1. Field of the Invention
This invention relates to control apparatus for use with numerically controlled drilling machines; and, more particularly, to a system for detecting and signaling the occurrence of broken or missing Drill Bits.
2. State of the Prior Art
Numerically controlled machine tools have been used for some time to increase the efficiency of generation of precision parts. It has been recognized that monitoring of the operation of such tools leads to an even greater efficiency in production of parts due to the minimization of preparation of faulty parts through a malfunction of the machine tool, and repair of malfunctions as they occur.
Numerically controlled Drilling Machines in general have control mechanisms for controlling the movement of Drill Chucks in relationship to a workpiece to be drilled, and control apparatus for positioning the workpiece that is being drilled to the precise location where the drilling operation is to occur. It is common to have the drilling machines operate in cycles wherein the control positions the workpiece, actuates the drilling operation, and signals a completion of the drilling operation, thereby ending that drilling cycle. The next drilling cycle can then be initiated, as determined by the numerical control signals applied.
The size of the holes to be drilled can vary over a wide range. For example, in the electronics industry, drilling of parts characteristically extends over the range of hole sizes of 30 mils in diameter to hole sizes in excess of 0.25 inch in diameter. In other industries and technologies, obviously even larger hole sizes are drilled. As the effort for further miniaturization in the electronics industry has progressed, it has been found to be necessary to drill holes of less than 30 mils in diameter. As integrated circuitry packages have been decreased in size it has become necessary to drill holes in the range of 1 mil to 20 mils in diameter for purposes of mounting the integrated circuit leads to printed circuit boards. Further, with decrease in circuit size, there has occurred an increase in the density of circuit mounting to multi-layer printed circuit boards. For example, it is common to have multi-layer printed circuit boards that are approximately one foot by one foot square, having as many as 20,000 holes drilled therein.
The hole location and drilling control instructions are often formed by computer program, and do not necessarily result in the holes being drilled in uniform pattern, nor in immediate progressive sequence. That is, holes may be drilled in various sections of the printed circuit board in sequential drilling operations. Accordingly, if a Drill Bit breaks and it is not detected, the workpiece may be moved to a position that renders it virtually impossible to back up to the point where the last good hole was drilled to again proceed once the Drill Bit has been replaced. Because of this repositioning problem, one alternative is to simply scrap the printed circuit board that is not completely drilled without breaking a Drill Bit. This obviously is an expensive alternative. In these extremely small diameter drill operations, it often occurs that just the tip of the bit breaks off. Since the Drilling Machine often utilizes a pressure foot to engage the workpiece before the drill is applied for drilling the hole, the Drill Bit is in a position that makes it very difficult to view visually, and to the extent that it can be viewed, it is difficult to determine visually when only a small portion of the Drill Bit has broken off.
Various attempts at solving the detection of broken Drill Bits in this extremely small size range utilizing concepts that work adequately for larger Drill Bits have been tried and found unworkable.
One attempted solution was to encircle the Drill Bit with coils of wire coupled to circuits attempting to sense the difference in inductance that occurs between the presence of a Drill Bit and the absence thereof. Due to the very small sizes of the drills and the fact that very small portions thereof may break off, inadequate signal changes for reliable detection were available. Further, proximity coils that would adequately accommodate the varying sizes of Drill Bits would render the system unacceptable.
Standard incandescent light sources with photoelectric interrupters were also considered. Again, the very fine drill size resulted in inadequate sensing of the interruption. Further, the portions of the Drilling Machine including the pressure foot, the spindle, the drive, and the like that are in the area of sensing would have to be coated with material to minimize reflection in order for the system to work. It was found also that when the operator checked the work operation using a flashlight to obtain additional illumination, the photoelectric system tended to malfunction.
In drilling systems utilizing larger Drill Bits, the power driving the drive motor can be sensed and the change in electrical load detected when the drill bit entered the workpiece. If the Drill Bit is broken or missing, there is no change in the motor load, thereby indicating a broken or missing Drill Bit condition. In practical operation, Drill Bits of the size contemplated do not produce sufficient load change to be reliably monitored for stopping operation for a broken Drill Bit.
Another system utilized with larger Drill Bits is a mechanical detection finger that simply is caused to move forward at the start of the drilling operation until it impinges on the Drill Bit. If stopped by the Drill Bit, it senses a good or drilling condition and the cycle is allowed to continue. If the Drill Bit is broken, the sensing mechanical feeler extends farther and indicates that the Drill Bit was broken. Mechanical sensing devices of this type actually tend to break small Drill Bits, and being mechanical in nature are difficult to mount on the numerically controlled Drilling Machine pressure foot, hence are unacceptable solutions to the problem.
Still another system considered was the use of the Drill Bit and the Conductor on the printed circuit board as a switch assembly. To accomplish this, the Drill Bit must be electrically isolated from the printed circuit board. A sensing circuit applied to the Drill Bit and the Conductor surface on the printed circuit board would then indicate a switching condition if the Drill Bit pierced the Conductor on the printed circuit board. Due to the extremely small diameter of the Drill Bit, a buildup of the epoxy being drilled would essentially isolate the Drill Bit from the Conductor, thereby tending to give false indications that the Drill Bit is missing.
For each of the foregoing items mentioned, the situation is particularly aggravated by the vibration and the contamination of the drilling environment.